Database Volumes

Launching Postgres

1. Download a postgres image, and look at its history to determine its default volume usage.

$ docker pull postgres:9
$ docker image inspect postgres:9

You should see a Volume block like the above, indicating that those paths in the container filesystem will get volumes automatically mounted to them when a container is started based on this image.

2. Set up a running instance of postgres container:

$ docker container run --name some-postgres \
    -v db_backing:/var/lib/postgresql/data \
    -d postgres:9

Notice the explicit mount, -v db_backing:/var/lib/postgresql/data; if we had'nt done this, a randomly named volume would have been mounted to the container's /var/lib/postgresql/data. Naming the volume explicitly is a best practice that will become useful when we start mounting this volume in multiple containers.

Writing on the database

1. The psql command line interface comes packaged with the postgres image; spawn it as a child process in your postgres container interactively, to create a postgres terminal:

$ docker container exec \
    -it some-postgres psql -U postgres

2. Create an arbitrary table in the database:

postgres=# CREATE TABLE CATICECREAM(COAT TEXT, ICECREAM TEXT);
postgres=# INSERT INTO CATICECREAM VALUES('calico', 'strawberry');
postgres=# INSERT INTO CATICECREAM VALUES('tabby', 'lemon');

Double check you created the table you expected, and then quit this container:

postgres=# SELECT * FROM CATICECREAM;
...
postgres=# \q

3. Delete the postgres container:

$ docker container rm -f some-postgres

4. Create a new postgres container, mounting the db_backing volume just like last time:

$ docker container run \
    --name some-postgres \
    -v db_backing:/var/lib/postgresql/data \
    -d postgres:9

5. Reconnect a psql interface to your database, also like before:

$ docker container exec \
    -it some-postgres psql -U postgres

6. List the content of the CATICECREAM table:

postgres=# SELECT * FROM CATICECREAM

The contents of the database have survived the deletion and recreation of the database container; this would not have been true if the database was keeping its data in the writable container layer. As above, use \q to quit the postgres prompt.

Running Multiple Database Container

1. Create another postgres runtime, mounting the same backing volume:

$ docker container run \
    --name another-postgres \
    -v db_backing:/var/lib/postgresql/data \
    -d postgres:9

2. Create another postgres interactive prompt, pointing at this new postgres container:

$ docker container exec \
    -it another-postgres psql -U postgres

Whenever data needs to live longer than the lifecycle of a container, it should be pushed out to a volume outside the container's filesystem, numerous popular databases are containerized using this pattern. In addition to making sure the data survives container deletion, this pattern allows us to share data among multiple containers, so multiple database instances can access the same underlying data.